1. Field of the Invention
The present invention deals with electrode tools for electroerosion (EDM) where a cavity or a relief is machined by hollowing out the part at great speed and in successive layers or slices (i.e. if "peelpocketting") by means of a rotating electrode tool of a simple shape which is independent of the cavity to be eroded, called "EDM slices milling". The fastidious designing and manufacturing of electrodes of complex shapes is thus avoided, thanks to an automatic breakdown of the geometry and to a machining simulation.
2. Background Information
This EDM slice milling method with wear compensation has already been described in the European patent application EP 555818 filed by the applicant. It concerns machining at a high wear rate where the length of a simply shaped electrode tool decreases rapidly, but without any apparent lateral wear. The longitudinal wear of the electrode tool is compensated by a regular, coaxial additional advance, directed of course towards the workpiece and corresponding to a theoretical value of that wear. The value of this advance is deduced by calculation based on technological preexisting data and on the geometry of the electrode tool, in such a way so as to keep the active end of the electrode tool parallel to the plane of the slice to be hollowed out. Moreover, the geometry of the three-dimensional volume to be hollowed out has been memorized as a superposition of virtually parallel slices (or layers). The active end of the electrode tool makes to and fro movements, or snaking movements or other sweeping movements, in the plane of each of these slices so as to erode them successively down to the bottom of the cavity to be machined.
Since EDM slices milling is a machining method with a high wear rate, the applicant has chosen, at a first stage, to use solid electrodes, in particular cylindrical ones, rather than hollow tubes. This choice corresponds to the most recent state of the art, that is to say the work done by M. M. Tsuchiya, T. Kaneko and S.Shoda (in particular "Three-dimensionally controlled EDM Using Column-Shaped Electrodes; J. Japan soc. Electrical Machining Eng. 1983, 17 (34), 30 42") .
Tubular electrodes, at first sight, have as many drawbacks as advantages. As a matter of fact, it did not really seem appropriate to use a tubular tool which gets worn out much faster than a solid one in spite of its known advantage offering the possibility of tracing grooves with a flat bottom. Indeed, as stated in Applicant's EP 555818, after a short transitional phase, the active end of the electrode tool takes an invariable profile called an "asymptotic or stationary shape". As illustrated in FIG. 1, the stationary shape of the tool tip 100 in the case of a rotating hollow tube 120 is a truncated cone 130. Using the tool tip 100, it is possible to trace adjacent grooves 140 with a flat bottoms 150. This facilitates erosion of successive flat slices, each being of a constant thickness, E wherein the thickness of the wall of the electrode is .tau.. The other known advantage of using tubular electrode tools in EDM machining is when a good irrigation of the machining gap is necessary by injecting or aspiring the dielectric through the hollow tube. In the case of EDM slices, such milling is only of little use, since the cavity to be machined is much larger than the tool.
The applicant has discovered quite unexpectedly that, thanks to the use of tubular electrodes, computer aided preparation of machining of EDM slices milling, as described in applicant's parallel application filed concurrently herewith (Attorney Docket No. 032530-007 (U.S. Ser. No. 08/780,127), having priority on Application No. 02 484/93-9 filed Aug. 20, 1993 in Switzerland), becomes possible. Such preparation, (e.g., of the programming of the plane paths at zero wear of the tool for each slice, the wear compensation, or the simulation of machining by calculating the material removal and by programming the machining speeds along these plane paths) seemed utopian and hardly implementable in view of the complexity subsequent to the accumulation of all of the differences between reality and the ideal universe of simulation. The tubular electrodes thus do not only possess as the sole advantage that of machining flat bottoms. Tubular electrodes have also made it possible to formulate, without any risk for the geometry of the finished part, simplified hypotheses to program computer aided preparation of EDM slices milling, and to envisage the calculation of theoretical material removal and machining speed at each point of the plane paths at zero wear.